Boosting uranium extraction from Seawater by micro-redox reactors anchored in a seaweed-like adsorbent

The study addresses the challenge of sustainably supplying uranium fuel for nuclear energy by extracting uranium from seawater, which contains an enormous but ultra-dilute resource (~3.3 ppb U; ~4.5 billion tons total). Traditional adsorbents (carbon materials, biomaterials, polymers, COFs, MOFs) face limited utilization of binding sites; once accessible sites (notably amidoxime groups) are occupied by U(VI), further adsorption is hindered by Coulomb repulsion and steric effects, resulting in actual amidoxime utilization rates often below 1%. Additional obstacles include the need for external energy for photo/electro-assisted systems and practical deployment issues for powder materials, as well as biofouling by marine microorganisms that block sites and degrade adsorbents. The research aims to enhance adsorption capacity by regenerating binding sites in situ and to improve anti-biofouling performance without external energy input.

Prior efforts to boost uranium uptake incorporated photocatalytic or electrocatalytic platforms into MOFs/COFs, achieving higher capacities but requiring external energy or being climate-sensitive and often in powder form that is hard to deploy at sea. Anti-biofouling strategies introduced components such as black phosphorus nanosheets, zwitterions, and quaternary ammonium ions to inhibit bacterial growth, yet many require external power or use weakly-bound components prone to detachment and secondary pollution. Amidoxime-bearing adsorbents dominate due to high affinity to U(VI), but practical site utilization is typically <1%, limiting capacity. Competitive ions in seawater (e.g., V(V)) can outcompete U(VI) for amidoxime binding, further challenging selectivity.

Materials synthesis: (1) Copolymerization of acrylonitrile (AN) and acrylamide (AM) using AIBN initiator in DMF at 65 °C for 7 h to obtain P(AN-co-AM) with varying AM content. (2) Amidoximation of P(AN-co-AM) with NH2OH·HCl and triethylamine in DMF at 65 °C for 16 h to yield P(AO-co-AM). (3) Self-assembly/crosslinking: Mix P(AO-co-AM) (in 0.1 M NaOH) with carboxymethyl chitosan (CMCS, in water), then introduce the mixture into a Cu(NO3)2 solution to instantly form seaweed-like balls or strings of CMCS/P(AO-co-AM)-Cu via coordination crosslinking. Variants with other metal linkers (+2, +3, +4 valences; e.g., Ca2+, Zn2+, Pr3+, Er3+, Zr4+) were prepared analogously. Activation by NaOH was applied to enhance hydrophilicity and porosity. Characterization: HAADF-STEM to confirm atomically dispersed Cu single atoms; EXAFS/WT-EXAFS for local coordination (≈3.1 N/O at 1.9 Å; no Cu–Cu signal); SEM/TEM/HRTEM for morphology and post-adsorption UO2 aggregates; EDS mapping; FT-IR and 13C/1H NMR to verify polymer structures; N2 adsorption–desorption (BET); TGA; mechanical testing (stress–strain, cyclic compression, rebound); dynamic water contact angle; XRD; XPS (U 4f, Cu 2p); EPR (U(IV) signal; ROS detection with DMPO/TEMP); XANES at Cu K-edge; zeta potential; electrochemistry (cyclic voltammetry vs SHE); ICP-MS/UV–vis for ion quantification. Adsorption tests: Batch adsorption in U-spiked seawater at initial concentrations of 8, 16, and 32 ppm (m=10 mg adsorbent; V=1 L; T=35 °C; pH optimized at 6). pH dependence (zeta potential correlation), thermodynamics (ΔH, ΔS, ΔG), kinetics (pseudo-first/second order, intraparticle diffusion), and isotherms (Langmuir). Natural seawater exposures without external energy input up to 56 days to determine time-dependent uptake. Selectivity assessed versus coexisting seawater ions, notably V. Comparative studies: Adsorbents crosslinked with non-redoxable Zn2+ prepared to isolate redox contribution; multiple other metal linkers evaluated to assess effect of valence and redox properties. Reusability/desorption: Uranium stripping using 0.2 M H2O2 + 1.0 M Na2CO3; five adsorption–desorption cycles. Continuous-flow column test with 100 ppm U-spiked seawater at 80 mL min−1, followed by in-column elution. Degradability in 0.1 M HCl. Antibacterial tests: ROS generation quantified by EPR; antibacterial efficacy against E. coli, Bacillus subtilis, Pseudomonas aeruginosa, and Staphylococcus aureus compared between Cu- and Zn-linked adsorbents.

• The Cu(I)/Cu(II) micro-redox reactors embedded in CMCS/P(AO-co-AM)-Cu regenerate binding sites by reducing adsorbed U(VI) to insoluble UO2 (U(IV)), enabling continued adsorption and increasing site utilization by >25% over a non-redox Zn-linked control.
• Adsorption capacities in U-spiked seawater (35 °C, pH 6; m=10 mg; V=1 L): 438.20 mg g−1 (8 ppm), 649.59 mg g−1 (16 ppm), and 962.40 mg g−1 (32 ppm).
• Natural seawater performance (no external energy): 6.21 mg g−1 after 1 week; 14.62 mg g−1 after 56 days, exceeding many reported porous polymers, fibers, and nanoparticles.
• Kinetics: Pseudo-second-order provides best fit with Q2,cal ≈ 446.61, 665.53, 980.39 mg g−1 closely matching experimental values, indicating chemisorption; intraparticle diffusion shows multi-stage transport with external mass transfer also contributing.
• Isotherm: Langmuir model with R2 = 0.999, indicating monolayer adsorption.
• Thermodynamics: ΔH > 0, ΔS > 0, ΔG < 0, indicating an endothermic, spontaneous process with increased entropy; higher temperatures improve uptake.
• Selectivity: U/V selectivity factor ≈ 2.6 after 56-day natural seawater adsorption; synergy of amidoxime and carboxyl groups plus reductive capture enhances U over V.
• Comparison with non-redox Zn-linked analog: CMCS/P(AO-co-AM)-Cu shows 26.8% (8 ppm) and 25.3% (32 ppm) higher capacity and slightly faster kinetics.
• Reusability/desorption: 98% U stripped within 1 min by 0.2 M H2O2 + 1.0 M Na2CO3; 86.6% capacity retained after 5 cycles; dynamic column test confirms adsorption and rapid elution; material fully degraded in 0.1 M HCl within <1 h.
• Antibacterial/anti-biofouling: Strong ROS (O2•− and 1O2) generation leads to high inactivation rates: E. coli 99.8%, B. subtilis 89.5%, P. aeruginosa 83.2%, S. aureus 81.1%; Zn-linked control shows markedly lower antibacterial performance.
• Structural/mechanistic evidence: Atomically dispersed Cu single atoms (HAADF-STEM/EXAFS); post-adsorption formation of UO2 aggregates (TEM/HRTEM: 0.19 nm lattice; XRD signature); EPR signal at g=1.99 (U(IV)); XPS shows U(IV) and U(VI) peaks; Cu 2p shifts and decreased Cu(I)/increased Cu(II) after adsorption; Cu K-edge XANES shifts consistent with higher Cu(II) after adsorption; CV onset oxidation potential ≈ −0.627 V vs SHE supports feasibility of U(VI) reduction.

The findings demonstrate that integrating Cu(I)/Cu(II) micro-redox reactors into a CMCS/P(AO-co-AM) matrix overcomes the low effective site utilization that limits conventional amidoxime-based sorbents. By reducing adsorbed U(VI) to insoluble UO2, the system releases and reactivates amidoxime and carboxyl binding sites, sustaining adsorption and delivering markedly higher capacities without external energy input. Mechanistic studies (EPR, XPS for U 4f and Cu 2p, Cu K-edge XANES, CV potential, TEM/HRTEM/XRD) corroborate in situ U(VI) reduction and Cu redox cycling during adsorption. DFT suggests a two-step pathway: Cu+-mediated reduction of U(VI) to U(V), followed by disproportionation to U(IV) (UO2) and U(VI), rationalizing experimental observations. The ROS generated by Cu redox chemistry confers strong antibiofouling, maintaining accessibility of sites in marine environments, which is crucial for long-term seawater deployment. Overall, the approach addresses key bottlenecks—site utilization and biofouling—positioning the material among top-performing sorbents for uranium recovery from seawater.

A seaweed-like, mechanically robust adsorbent (CMCS/P(AO-co-AM)-Cu) was developed that self-assembles rapidly and embeds atomically dispersed, valence-variable Cu centers functioning as micro-redox reactors. This design enables redox-assisted regeneration of adsorption sites and ROS-mediated antibiofouling, achieving high uranium capacities in spiked seawater (up to 962.40 mg g−1) and leading natural seawater uptake (14.62 mg g−1 in 56 days), with excellent selectivity, fast and efficient desorption (98% in 1 min), reusability (86.6% retention after 5 cycles), and easy end-of-life degradation. The strategy provides a scalable, energy-independent pathway for efficient uranium extraction from seawater. Future work should focus on larger-scale and long-duration ocean demonstrations, optimization of Cu loading and distribution, and engineering of deployment systems for practical marine conditions.

• Activation with NaOH is required to achieve efficient uranium capture, indicating a necessary pretreatment step.
• Reduction of Cu(II) to Cu(I) by amidoxime-containing reagents can consume some amidoxime groups, presenting a trade-off between redox functionality and ligand availability (though the gains in adsorption offset this loss).
• Natural seawater results, while strong, were measured over 56 days and at site-specific conditions; broader, long-term ocean trials are needed to fully assess performance and generalizability.